docs: def start_tag(self, name, attrs=None):
          """Open an XML tag"""
          if not attrs:
              self._write('<%s>' % name)
          else:
              self._write('<' + name)
              for (name, value) in sorted(attrs.items()):
                  self._write(
                      ' %s=%s' % (name, quoteattr(scientificformat(value))))
              self._write('>')
          self.indentlevel += 1